1. Field of the Invention
The present invention relates to an integrated circuit for an in-vehicle network communication system such as a CAN system or a FlexRay system or all communication systems to which an overvoltage may be applied.
2. Description of the Related Art
In an in-vehicle network for performing various controls of a car, a controller area network (CAN) and a local interconnect network (LIN) are now prevailed as world standards and used for body control, temperature adjustment control, dashboard control, navigation, various sensor controls, motor control, chassis control, and the like. In contrast, there is a next-generation in-vehicle communication protocol FlexRay system. The FlexRay system has a feature in that the reliability of data transmission is higher than that in the CAN. To be specific, a data transmission rate is 10 Mbits/seconds in maximum which corresponds to ten times that of the CAN. A time-triggered system with little delay is employed for data transmission. Two control large-scale integration (LSI) circuits and two transmission and reception LSI circuits per node are mounted and double transmission media are provided, thereby improving the reliability.
In order to prevent an LSI circuit from being broken by a noise applied thereto, a circuit for protecting the LSI circuit from an overvoltage is provided in an output circuit such as a FlexRay bus driver. An example of a conventional overvoltage damping circuit for two-wire bus system is described in JP 2002-532961 A. FIG. 8 shows the damping circuit described in JP 2002-532961 A. In order to damp a level of an overvoltage applied by resonance, Zener diodes are used in the damping circuit. To be specific, as shown in FIG. 8, when an overvoltage “A” generated by high-frequency (HF) radiation with respect to a reference potential 210 is applied to nodes 207, 208, and 209, resistors 203 and 204 are connected with the reference potential at a potential (each of forward and backward voltage) in which Zener diodes 205 and 206 are turned on, thereby damping a level of the overvoltage applied to the nodes 207, 208, and 209.
In JP 2002-532961 A, the Zener diodes are used to prevent the application of the overvoltage. However, it is difficult to embed a Zener diode having a low voltage of, for example, approximately 7 V in the LSI circuit owing to the severe limitation of process. Therefore, it is expected to construct an overvoltage reduction and protection circuit using high-withstand voltage MOS transistors instead of the Zener diodes.
A “OUTPUT CIRCUIT AND PROTECTION CIRCUIT” section of FIG. 9 is a circuit diagram showing an example in which the overvoltage reduction and protection circuit is realized without the use of the Zener diodes. When high-withstand voltage transistors and high-withstand voltage resistors are used instead of the Zener diodes, the high-withstand voltage transistors and the high-withstand voltage resistors can be embedded in the LSI circuit. The operational principle is substantially identical to that in the case where the Zener diodes are used.
As shown in FIG. 9, overvoltage reduction and protection circuits 120P and 120M are provided to output wirings for differential signals BP and BM from an output circuit 110. The overvoltage reduction and protection circuit 120P includes a Pch-transistor P101 and a resistor R101 which are connected in series between a power supply wiring and one of the output wirings. The overvoltage reduction and protection circuit 120P further includes a resistor R102 and an Nch-transistor N101 which are connected between the one of the output wirings and a ground wiring. Similarly, the overvoltage reduction and protection circuit 120M includes a Pch-transistor P102, a resistor R103, a resistor R104, and an Nch-transistor N102.
When a potential of each of the differential signals BP and BM is within a range of 0 V to 5 V, currents do not flow into the overvoltage reduction and protection circuits 120P and 120M. That is, the normal operations of the overvoltage reduction and protection circuits 120P and 120M are not influenced. When the potential of each of the differential signals BP and BM becomes equal to or larger than approximately 5.7 V, a diode formed between substrates and the Pch-transistors P101 and P102 is turned on. Then, a current (hereinafter referred to as substrate current) corresponding to each, of the differential signals BP and BM flows in a power supply wiring direction (through each path indicated by reference numeral 131 in FIG. 9). Therefore, only when a positive noise is applied, a common mode impedance reduces.
When the potential of each of the differential signals BP and BM becomes equal to or smaller than approximately −0.7 V, a diode formed between substrates of the Nch-transistors is turned on. Then, a substrate current corresponding to each of the differential signals BP and BM flows in a ground wiring direction (through each path indicated by reference numeral 132 in FIG. 9). Therefore, only when a negative noise is applied, a common mode impedance reduces.
A “BP, BM VOLTAGE WAVE PATTERN” of FIG. 9 shows the swing of the potential of each of the differential signals BP and BM. Before and after measures, a common mode swing width of each of the differential signals BP and BM narrows. The conventional overvoltage reduction and protection circuit has a merit in that the circuit is easily embedded in the LSI circuit because it is unnecessary to use the Zener diode having the low voltage.
For example, in the case of the FlexRay system, a specification of the FlexRay defines a limitation of a bus leakage current. In the specification, a current flowing into each of the output wirings for differential signals BP and BM at VCC=0 V and BP=BM=5 V is equal to or smaller than 25 μA. However, according to the circuit structure shown in FIG. 9, a current equal to or larger than the limitation of bus leakage current flows into each of the substrates of the Pch-transistors.
That is, in the conventional overvoltage reduction and protection circuit shown in FIG. 9, the protection Pch-transistors are provided on the VCC side. Therefore, when a power supply for the LSI circuit is turned off and thus a constant potential is being applied to buses, the current (substrate current) flows from the buses through the diode formed between the substrates and the Pch-transistors. Then, as described above, a current is significantly deviated from the specification in the FlexRay system. Thus, when the power supply for the LSI circuit is turned off, the current flowing from the buses increases, so it is difficult to realize overvoltage protection in a system in which the bus leakage current is limited, such as the FlexRay system.
As described above, it is difficult to embed the Zener diode in the LSI circuit owing to the severe limitation of process. Therefore, a circuit capable of cutting off an inflow current at the time of turning OFF the power supply while the overvoltage protection is realized using MOS transistors is required.